The present invention relates to propellers for aircraft, and in particular the present invention relates to a series of components which may be assembled to make propellers of different pitches and different diameters suitable for any of a wide range of ultralight aircraft.
Ultralight aircraft typically are made of tubing and wire rigging over which fabric is stretched to form air foils. To this air frame a small engine is attached. The engine is usually no more than two cylinders and less than about 40 horsepower. It drives a propeller either directly or through a reduction drive or transmission. These engines are tuned to produce maximum horsepower over a relatively narrow rpm range. The range of maximum power rpm at the output of a reduction drive is even narrower. Therefore the propeller must be matched to the engine, reduction drive, and air frame to allow the engine and propeller both to operate at maximum power.
Propellers for ultralight aircraft have traditionally been made of wood, usually laminated birch or walnut, and have been made by hand. Although wood has the advantage of being light and relatively strong, it also has inherent disadvantages. Wood is subject to warpage with changing temperature and humidity. Additionally, the density of wood is not uniform which means that two blades of a wood propeller, even if they have identical shapes, might not balance each other either statically or dynamically. As a consequence, wooden propellers have been relatively expensive, and when one blade is damaged, the entire unit must be replaced. Moreover, the replacement propeller, although supposedly the same as the damaged one, will never be identical to it and so identical performance cannot be assured.
Additionally, fitting a propeller to an ultralight aircraft is as much art as science. The interaction of engine, transmission, air frame, and propeller include too many variables to be completely understood from a theoretical approach. Although general guides are known for selecting a propeller of the proper pitch and diameter, in the final analysis trial and error is the only known method of selecting a propeller that will maximize performance.